In an interferometric measuring device described in published German patent document DE 100 57 539, the interferometric measuring device is subdivided, on the one hand, into a modulation interferometer and, on the other hand, into a measuring probe having an additional interferometer unit. In the measuring probe there is provided a probe-optical fiber unit having an exit surface at the object end which, for instance, may be beveled. By the way, such an interferometric measuring device works in a manner described in greater detail in published German Patent document DE 198 19 762.
In an additional interferometric measuring device described in published German patent document DE 198 19 762, one part, the so-called modulation interferometer, is spatially separated from the actual measuring probe, and is optically connected to it via a light-conducting fiber system, so that the measuring probe may be designed as a relatively simply constructed, easily manipulable unit. A broad-band, short-coherent radiation is supplied to the modulation interferometer, which is split into two beam components at the input of the modulation interferometer with the aid of a beam splitter, of which the one is shifted in its light phase or light frequency with respect to the other, using a modulation device, such as an acousto-optic modulator. In the modulation interferometer, one of the two beam components runs through a delay element which generates an optical path difference of the two beam components which is greater than the coherence length of the short-coherent radiation. In the measuring probe, in a measuring arm, with respect to a reference arm, an additional optical path difference is generated in such a way that the path difference effected by the delay element is compensated for, and, consequently, an interference is created between the reference radiation coming from the reference plane of the reference arm and the radiation coming back from the object surface in the measuring arm, which interference is subsequently analyzed so as to ascertain the desired surface property (shape, roughness, clearance distance) via a phase evaluation. In the measuring probe, the measuring arm and the reference arm are situated in one exemplary embodiment in one common light path (common path), a partially transmitting optical element being provided for forming the measuring arm and the reference arm.
A similar interferometric measuring device having such a modulation interferometer and a measuring probe connected to it via a light-conducting fiber system is also described in published German patent document DE 198 08 273. In a beam splitting and radiation detecting unit, using a receiving equipment, the radiation brought to interference is split into radiation components of different wavelength, so as to form therefrom a synthetic wavelength and to increase the measuring range.
In the interferometric measuring devices named above, which are based on heterodyne interferometry, but which utilize the properties of a broad-band, short-coherent radiation, the modulation interferometer, designed as a Mach-Zehnder interferometer, has a system of classical optical components, such as collimation optics lying upstream of the input end of the beam splitter, with the beam splitter and reflecting mirror at the input end and the output end, respectively. In this context, the beam components experience several reflections at the beam splitter surfaces and at the mirrors, before they are coupled into the optical light-conducting fiber system. The optical components have to be positioned with great accuracy, since the effect of every angle error is doubled by the reflection. In this context, it is difficult to ensure the durability of a calibration. In connection with fitting in a glass plate to compensate for optical asymmetries, additional difficulties come about during the calibration. A costly construction is connected with these difficulties, an exact adjustment to the properties of the measuring probe being also required.
An object of the present invention is making available an interferometric measuring device which permits achieving as accurate as possible a measurement, using a simplified construction.